Discharge lamps, such as high-pressure mercury lamps or metal halide lamps, have been used as light sources of a projector. In these discharge lamps, the shape of an electrode changes due to a drop in fusibility resulting from consumption of the electrode by discharge or progress of crystallization of the electrode according to an increase in cumulative lighting time. In addition, when a plurality of projections grow in an electrode tip portion or irregular consumption of the electrode body progresses by those described above, the arc origin moves or the arc length changes. Such phenomena are not desirable because they reduce the brightness of a discharge lamp so that the lifespan of the discharge lamp is reduced.
In order to solve the problem, a discharge lamp lighting device that drives a discharge lamp using alternating currents with different frequencies is known (see, e.g., JP-A-2006-59790). In addition, a discharge lamp lighting device that supplies a driving current, in which a direct current is intermittently inserted in a high-frequency alternating current, to a discharge lamp is known (see, e.g., JP-A-1-112698).
However, if the thermal conditions (easiness of temperature increase or temperature decrease) of electrodes provided in a discharge lamp are different for every electrode, the degree of growth or melting of a projection of an electrode tip portion which becomes an arc origin changes with an electrode. For this reason, particular consideration is required in order to maintain the shape of the projection of the electrode tip portion. For example, a projection of an electrode tip portion under the condition where the temperature thereof is likely to rise may disappear. If the projection of the electrode tip portion disappears, the arc origin becomes unstable or it becomes a cause of further deformation of the electrode. Moreover, an electrode material evaporates excessively from the electrode tip portion which is under the condition where the temperature is likely to rise. As a result, blackening, which occurs by adhesion of the evaporated electrode material to the sealed body, and formation of needle-like crystals may progress more easily.